There are a number of different classes of sol-gel process for the preparation of porous oxide bodies known in the prior art. In one such class the dispersion medium (which is often water) is extracted from the sol particles by a medium having greater avidity therefor, resulting in their gelling. Examples of extraction media include higher anhydrous alcohols (as in the Oak Ridge process) or concentrated aqueous ammonia. The former example involves separation problems in that organic residues can only be removed from the gel particles with difficulty and in that the dispersion medium has subsequently to be separated from the extraction medium. The latter example results, in general, in a gelatinous shell forming round the remainder of the sol particle which either collapses or ruptures, both with loss of dimensional integrity, depending on the resulting osmotic stress.
In another class the dispersion medium is extracted from the sol particles by subliming the dispersion medium by freeze-drying. In U.S. Pat. No. 3,551,533 there is disclosed a process wherein a solution of a solute material, for example aluminium sulphate, is broken up into droplets which are rapidly frozen to prevent coalescence and the solvent is removed by sublimation to provide porous dried products, for example aluminium oxide, suitable for use in abrasives. Examples of refrigerating fluids disclosed include liquid nitrogen. U.S. Pat. No. 3,422,167 discloses a process for the preparation of metal oxide microspheres by dispersing a corresponding metal oxide sol into a freezing medium and subsequently dehydrating the sol by vacuum distillation, and then calcining.
This invention seeks to provide an improved process for preparing porous oxide and hydroxide bodies, for example microspheres, which is both simpler and cheaper than conventional processes and results in a cleaner product.